When the appearance of the face is of lesser concern than the strength and stability of the material, structural plywood will typically be the choice. The resins used to adhere the plies are designed for extra strength to avoid separating of the layers. Structural plywood is seldom found in a grade higher than C-D. This type of plywood is often referred to as CDX. The CDX Grade comes from US voluntary plywood Standard PS1-95 for Construction and Industrial Plywood established by APA – The Engineered Wood Association.
“CDX” is not the name of a plywood grade . CDX stands for “C-D Exposure 1 plywood” . C-D means that one side of the plywood is rated “C” grade and the other side is rated “D” grade . The letter “X” means the glue of the plywood is exterior glue . But CDX plywood is not exterior plywood . Because usually the core veneers for CDX plywood are not as good as exterior plywood. CDX plywood is moisture resistant but can not be exposed to water or weather for long time . CDX plywood is a kind of economical plywood . Usually CDX plywood are sanded but not polished . So the face/back of CDX plywood is rough . Some knots and knot holes exist in the face/back . As construction and industrial plywood, CDX plywood is usually used as wall sheathing, roofing and so on.
Below are some typical APA Stamps and their explanation.